This invention relates to electrostatic voltmeters, more commonly referred to as electrometers.
An electrometer is an instrument for measuring the electrostatic charge potential levels on a surface. Generally, a probe is placed near the test surface and the charge thereon is capacitivly coupled to the probe. Associated electrical circuitry electrically connected to the probe provides a voltage output representative of the quantitative charge sensed by the probe. Typically, this voltage output is read and displayed by a commonly used voltmeter.
While the electrometer may be used in a variety of applications, it has found increasing use in the xerographic process wherein copies are produced from a latent electrostatic image formed on a charged photoreceptor. To insure good copy quality, the level of charge on the photoreceptor must be maintained within predetermined limits. Consequently, service personnel must periodically check to see if the proper charge level is being maintained on the photoreceptor and, if not, they must adjust the amount of charge being applied to the photoreceptor, change exposure levels, etc. Normally, the service personnel also makes other adjustments to the machine. With the advent of present day highly complex machines, there are many other technical measurements and adjustments to be monitored. Accordingly, the number of tools necessary for the service personnel to accomplish their tasks are proportionally increasing. Therefore, it is a feature of this present invention to provide an electrometer housing which is attachable to a voltmeter to thereby provide a compact and readily accessible tool for measuring charge levels on a surface.
Electrometers are extremely delicate instruments. Therefore, care must be taken to prevent spurious high voltage transients from entering the finely adjusted circuitry or else permanent damage may result to the components. Similarly, the charge receiving element and other components in the probe must be protected from adverse environmental conditions which can effect their ability to accurately measure the charge from the test surface. Unfortunately, users have not always given their electrometers the attention they deserve, with the result of repairs or replacement being often needed. Therefore, it is another feature of this invention to provide a probe which provides excellent protection to increase the accuracy and longevity of the electrometer.
In servicing xerographic machines, other voltage measurements must be taken on various other machine components which, unlike the photoreceptor, may be physically contacted with the wires of a voltmeter. The electrometer, instead of the voltmeter, is used in measuring the photoreceptor potential because the wires or clips of the voltmeter could scratch its surface. However, other components such as power supplies can be contacted with the voltmeter and a direct reading may be taken without the use of the electrometer. Unfortunately, inaccurate readings have been experienced due to the voltmeter "loading down" the supply. To alleviate this condition, this invention provides an adapter for the electrometer probe which permits accurate contact measurements to be made. Since by using the housing of this invention the service personnel already has his electrometer attached to the voltmeter, the more accurate reading can be made without unduly increasing the number of tools required.
Early d.c.-type electrometers suffered from spacing sensitivity; i.e., their output measurements would be critically dependent upon the distance between the probe and the test surface. It has been suggested to obviate this problem by feeding back to the probe shield a voltage equivalent to that sensed by the probe charge receiving element. It is a further feature of the present invention to improve upon this general concept and provide an electrometer with minimal power requirements, components, and current capacity. The present invention permits battery operation which further increases the utility of the device in a field type environment, e.g., increased portability, with the ability to measure "floating" voltages not referenced to earth ground. Moreover, the cost of the device is maintained at a relatively low level and the useful life of the electrometer is increased.
The electrometer of this invention also includes the capability of self-testing the operation of its components, as well as automatically zeroing or calibrating itself prior to use to insure accurate measurements .